We report the results of isotopic, chemical, structural, and crystallographic microanalyses of graphitic spherules (0.3È9 km) extracted from the Murchison meteorite. The spherules have 12C/13C ratios ranging over 3 orders of magnitude (from 0.02 to 80 times solar), clearly establishing their presolar origin as stellar condensates. These and other isotopic constraints point to a variety of stellar types as sources of the carbon, including low-mass asymptotic giant branch (AGB) stars and supernovae. Transmission electron microscopy (TEM) of ultrathin sections of the spherules revealed that many have a composite structure consisting of a core of nanocrystalline carbon surrounded by a mantle of well-graphitized carbon. The nanocrystalline cores are compact masses consisting of randomly oriented graphene sheets, from PAH-sized units up to sheets 3È4 nm in diameter, with little graphitic layering order. These sheets probably condensed as isolated particles that subsequently coalesced to form the cores, after which the surrounding graphitic mantles were added by vapor deposition. We also detected internal crystals of metal carbides in one-third of the spherules. These crystals (5È200 nm) have compositions ranging from nearly pure TiC to nearly pure Zr-Mo carbide. Some of these carbides occur at the centers of the spherules and are surrounded by well-graphitized carbon, having evidently served as heterogeneous nucleation centers for condensation of carbon. Others were entrained by carbon as the spherules grew. The chemical and textural evidence indicates that these carbides formed prior to carbon condensation, which indicates that the C/O ratios in the stellar sources were very close to unity. Only one of the 67 spherules studied in the TEM contained SiC, from which we infer that carbon condensation nearly always preceded SiC formation. This observation places stringent limits on the possible delay of graphite formation and is consistent with the predictions of equilibrium thermodynamics in the inferred range of pressure and C/O ratios. We model the formation of the observed refractory carbides under equilibrium conditions, both with and without s-process enrichment of Zr and Mo, and show that the chemical variation among internal crystals is consistent with the predicted equilibrium condensation sequence. The compositions of most of the Zr-Mo-Ti carbides require an s-process enrichment of both Zr and Mo to at least 30 times their solar abundances relative to Ti. However, to account for crystals in which Mo is also enriched relative to Zr, it is necessary to suppose that Zr is removed by separation of the earliest formed ZrC crystals from their parent gas. We also explore the formation constraints imposed by kinetics, equilibrium thermodynamics, and the observation of clusters of carbide crystals in some spherules, and conclude that relatively high formation pressures dynes cm~2), and/or condensable carbon number densities cm~3) are required. (Z0.1 (Z108 The graphite spherules with 12C/13C ratios less than the solar valu...

In order that the effect of graviation of the sion of the universe not be too severe, their

The observation of neutrons turning into antineutrons would constitute a discovery of fundamental importance for particle physics and cosmology. Observing the n−n transition would show that baryon number (B) is violated by two units and that matter containing neutrons is unstable. It would provide a clue to how the matter in our universe might have evolved from the B = 0 early universe. If seen at rates observable in foreseeable next-generation experiments, it might well help us understand the observed baryon asymmetry of the universe. A demonstration of the violation of B − L by 2 units would have a profound impact on our understanding of phenomena beyond the Standard Model of particle physics.Slow neutrons have kinetic energies of a few meV. By exploiting new slow neutron sources and optics technology developed for materials research, an optimized search for oscillations using free neutrons from a slow neutron moderator could improve existing limits on the free oscillation probability by at least three orders of magnitude. Such an experiment would deliver a slow neutron beam through a magnetically-shielded vacuum chamber to a thin annihilation target surrounded by a low-background antineutron annihilation detector. Antineutron annihilation in a target downstream of a free neutron beam is such a spectacular experimental signature that an essentially background-free search is possible. An authentic positive signal can be extinguished by a very small change in the ambient magnetic field in such an experiment. It is also possible to improve the sensitivity of neutron oscillation searches in nuclei using large underground detectors built mainly to search for proton decay and detect neutrinos. This paper summarizes the relevant theoretical developments, outlines some ideas to improve experimental searches for free neutron oscillations, and suggests avenues both for theoretical investigation and for future improvement in the experimental sensitivity.

We present a straightforward model of cosmic-ray propagation in the Galaxy that can account for the observed cosmic-ray positrons entirely as secondary products of cosmic-ray nucleons interacting with the interstellar medium. In addition to accounting for the observed energy dependence of the ratio of positrons to total electrons, this model can accommodate both the observed energy dependence of secondary to primary nuclei, like Boron/Carbon, and the observed bounds on the anisotropy of cosmic rays. This model also predicts the energy dependence of the positron fraction at energies higher than have been measured to date, with the ratio rising to ∼0.7 at very high energies. We briefly point out the differences between this model and the model currently in wide use that does not account for the observed positrons as secondaries and so prompts the interpretation of the observations as evidence for alternate origins of positrons.

Double beta decay of '2'Te has been confirmed and the ratio of half-lives for pp decay of " Te and Te has been precisely determined as Tl&2/T',~&=(3.52+0. 11)X10 by ion-counting mass spectrometry of Xe in ancient Te ores, using techniques that reduce interferences due to trapped Xe. We have also detected excesses of ' Xe originating in high energy reactions of cosmic ray muons and their secondaries on Te; such reactions make minor contributions to the measured ' Xe excesses in the Te ores. The Xe measurements, combined with common Pb dating of the ores, yield a ' Te half-life of (2.7+0. 1)X 10 ' yr and thus a ' Te half-life of (7.7+0.4) X 10 yr the longest radioactive decay lifetime measured to date. These results give limits on the effective Majorana mass of the neutrino ( & 1.1 -1.5 eV) and right-handed currents (~(g )~( 5.3 X 10 ') comparable to the best obtained from direct neutrinoless PP-decay searches. They also imply new limits on unconventional Majorons not constrained by measurements of the Z decay width. PACS number(s): 23.40.8w, 14.60.Gh, 14.80.Gt, 27.60.+j PRECISE DETERMINATION OF RELATIVE AND ABSOLUTE. . . 807 T0( m ) to T0 ( m ), where ( m ) is the effective Majorana mass of the neutrino [4,5]. This implies that the relative contribution of Ov /3P decay to the ' Te decay (TQ =0.87 MeV) is much larger than that for ' Te (T0 =2.53 MeV) for values of ( m ) near the current limits of a few eV. The experimental advantage of the Te system is that both of the daughters of the Te PP decay, Xe and ' Xe, are noble gases which can be easily extracted from Te ores. Their ratio, ' Xe/' Xe, is directly proportional to the ratio of decay rates I' /I', and can be determined with relatively high precision by noble-gas mass spectrometry, thus eliminating many sources of systematic error. These advantages provided by the Te system have prompted many groups to study it, and although the decay of ' Te is well established, there has been a longstanding controversy over whether PP decay of ' Te has actually been observed. Several studies by the University